1. Field of the Invention
The present invention relates to a process for preparing contact lenses made up of an interpenetrating polymer network (IPN) of polyurea and polyacrylic by the reaction injection molding process.
2. Discussion of the Prior Art
Contact lenses are precision ophthalmic devices. The manufacture of contact lenses requires the ability to produce a device with two curved surfaces to a radius of curvature with an accuracy of better than ten microns. The surfaces must be of optical quality, and the lens edges must be smooth with minimal defects.
With a growing preference for contact lenses as corrective ophthalmic devices, and the introduction of wearing modes requiring frequent replacement of such lenses, there exists a real need for a cost-effective, high speed process of manufacturing high quality precision lenses.
Historically, contact lenses have been prepared from polyhydroxyethylmethacrylate (PHEMA). Three manufacturing processes have generally been utilized: lathing, spincasting and cast molding. Unfortunately, each of these processes has its disadvantages associated with it which prevent it from producing high precision ophthalmic devices in an efficient and quick manner.
Lathing is slow and lacks precision for high speed production. Typically, a lens is manufactured to the best specifications. The parameters are determined for the finished lens. The finished lens is then "slotted" to its closest power and packaged for sale. However, the process of lathing is inadequate to meet the demands of high speed high volume production.
The technique of spin casting is capable of producing precision lenses. A liquid monomer containing cross-linking agents is injected into the concave cavity of a precision plastic mold, and the mold is spun at a predetermined rate. The resulting centrifugal force causes the liquid monomer to spread over the surface of the concave mold in the shape of a contact lens. The centrifugal force is modulated by the spinning rate of the plastic mold. Thus, by adjusting the spinning rate of the mold, contact lenses with precise thickness and prescription can be manufactured. The spin cast lens, thus prepared, requires that the edges be polished prior to hydration and released from the mold.
More specifically, in the spin-casting technique, the front surface of the contact lens is shaped by the mold, while the back surface is shaped by the free spinning of the mold. The centrifugal force generated by the spinning mold often produces spherical base curves in the back surface of the contact lens. The nature of these base curves can affect the optical quality and fitting of the finished lenses. While the technique of spin casting is capable of producing precision lenses, it is inadequate to meet the combined demands of high speed and cost-effective production.
Cast molding is an increasingly used manufacturing process for rigid gas permeable lens like siloxane rubbers, and for soft hydrogel contact lenses made from poly(hydroxy ethyl methacrylate). It is, thus far, the most successful high speed, high precision process for making contact lenses. It is a closed molding process. Cast molding requires the use of two molds with an annular contact, wherein the molds used are derived from a variety of plastics.
In cast molding, a female mold forms the lens' convex front surface, while a male mold forms the lens' concave back surface. The liquid monomer is placed in the cavity and sealed by the annular contact between the two molds. Polymerization occurs in the closed cavity, and the polymerized hardened lens is released from the mated molds. The subsequent processing of the lens is similar to the methods used for producing lens by spin casting.
Lenses produced by cast molding have a number of defects resulting from shrinkage due to polymerization. The negative entropy accompanying polymerization reactions leads to a reduction in the volume of the polymer, as compared to the volume of the starting monomer. This shrinkage in volume occurs inside the closed cavity of the two mated molds used in this process. The resulting lens frequently has surface voids and edge irregularities resulting in a higher than ideal fraction of unusable lenses. Various methods have been used to attempt to eliminate the shrinkage defects. For example, in U.S. Pat. No. 4,640,480, the molds are modified to ameliorate such shrinkage defects. Another technique is to use a diluent to mitigate the shrinkage effect during polymerization.
It is clear from the above discussion that there exists a real need for a contact lens manufacturing process that meets the combined objectives of cost-effectiveness, high speed production and high quality precision and minimizes the effects of shrinkage on the lens. In order to overcome the inadequacies of these various techniques, investigations have been made into the use of different materials for the preparation of contact lens. Considerable attention has been given to the modification of polymer properties through the use of procedures involving the formation of an interpenetrating polymer network (IPN).
An interpenetrating polymer network (IPN) is defined as an intimate combination of two or more polymers, both in network form, at least one of which is synthesized or cross-linked in the immediate presence of the other. In such a simultaneous synthesis, monomers of two or more different polymers are cross-linked and polymerized by non-interfering mechanisms. The crosslinking of at least one of the polymer systems distinguishes an IPN from a chemical blend. Such a physical combination of two or more structurally dissimilar polymers provides a convenient way of combining the different properties of individual polymers. A comprehensive review of Interpenetrating Polymer Networks is described in Vol. 8, Encyclopedia of Polymer Science and Engineering, pp 279-341 (1985), the contents of which are incorporated by reference. Current developmental efforts in IPN materials for contact lenses strive to combine the excellent mechanical properties of hydrophobic polymers with the soft, wettable and oxygen permeable properties of hydrophilic polymers.
Liu for example, in U.S. Pat. No. 4,618,644 describes the polymerization of methyl methacrylate in the presence of a silicone polymer to obtain a product of improved toughness. The polymerization of hydroxyethyl methacrylate in the presence of ethylene glycol dimethacrylate and a cross-linkabie poly (dimethylsiloxane) to yield a product stated to be useful for the fabrication of contact lenses is described by Falcetta (Ger. Offen. DE 2,518,904). Contact lenses have also been fabricated from the interpenetrating network polymer resulting from the polymerization of 2-hydroxyethyl methacrylate in the presence of poly-N-vinylpyrrolidone (Ewell, U.S. Pat. No. 3,647,736).
Neefe (U.S. Pat. No. 4,632,773) shows the polymerization of methyl methacrylate in the presence of a syrup containing polymerized methacryloxypropyl-trimethoxysilane and a fluorescent colored pigment to obtain a solid contact lens blank material which can be readily identified. Tighe and Gee (U.S. Pat. No. 4,430,458) disclose the formation of a soft contact lens material by the cross-linking of a polymeric hydrogel of a copolymer of N-vinyl-2-pyrrolidone during the final compression of injection molding process. Lim et al. (U.S. Pat. No. 4,536,554) describe the preparation of soft contact lenses made from the interpenetrating network polymer obtained by the polymerization of a mixture containing a hydrophilic and a hydrophobic monomer and at least two cross-linking agents.
But, even with the use of IPN networks in making contact lenses, the aforementioned techniques have been utilized. For example, cast molding has been utilized in U.S. Pat. No. 5,170,192 to Pettigrew et al. to prepare the bifocal contact lenses therein and to prepare the contact lenses described in U.S. Pat. No. 5,087,392 to Burke, et al.
Although the use of IPN's may improve the shrinking effect, it still has not overcome the problem completely. Moreover, the use of IPN's heretofore has not significantly reduced the manufacturing time for making the contact lens. For example, in U.S. Pat. No. 4,536,554 to Lim, et al., the copolymerization to form an IPN from vinyl pyrrolidone and 5-alkylene-m-dioxanyl acrylic ester took at least 6-8 hours.
Thus, there is still an unfilled need in the optical industry to prepare contact lenses that minimizes or substantially eliminates the shrinkage effects, and that manufactures contact lenses in a simple and economical procedure. It is towards this need that the present invention is directed.
The present inventors have found a solution that addresses this need. The present invention uses a Reaction Injection Molding (RIM) process to manufacture precision contact lenses.
The RIM process has been utilized in plastics technology and especially in the automotive industry especially in the manufacturing of bumpers and dashboards, but until the present invention, has never been utilized to prepare precision contact lenses. The general technique of utilizing the RIM process is described in an article by L. T. Manzione, in 14 Encyclopedia of Polymer Science and Technology, pp 74-100, the contents of which are incorporated herein by reference.
The RIM process is a polymer process operation wherein reactive low viscosity liquid components are mixed, typically by impingement, injected into a mold and polymerized therein to form a polymer article. A schematic of a typical RIM process is shown in FIGS. 1 and 2 for a two stream process. Typically, the RIM process proceeds as follows:
Referring to FIG. 1, the monomers (1) are stored in separate reservoirs or tanks (14), usually under nitrogen or dry air blankets (9). These tanks are jacketed and equipped with stirrers (2) in order to maintain the monomers at a specified temperature range. The monomers are in the liquid state in these tanks.
A predetermined and precise amount of each liquid component is drawn from each tank through an inlet line (4) with a metering cylinder or pump (5) and delivered to the mixing chamber, the mixhead (7), at high pressure through connecting means (10). A recirculation line (6) connects the mixhead with the storage tank line (3) through which excess material can be returned to the reservoir.
The high pressure is required to attain material velocity and turbulence sufficiently high to cause thorough mixing of the two monomers.
Before entering the mixhead, wherein the monomers meet, they pass through small apertures in the side wall of the mixing chamber. In a typical mixhead, the mixing chamber is created by the pullback of a rod (15), creating a cylindrical cavity. The orifices impinge the high pressure fluid streams at 180.degree. angle, frequently in the rear portion of the cavity. The rod is activated, at the conclusion of the impingement, and moves forward to push the reacting fluid from the chamber into a mold cavity (8), which is attached to the mixhead (see FIG. 2). The reactive material passes through a gate (13) into the mold wherein the polymerization is completed. Typically, to prevent trapping of air inside the mold, the mold contains a vent (12) which passes from the mold wall (11) to the mold cavity (8).
The present inventors have found a means of utilizing this RIM process for the manufacture of contact lenses from IPN material. This methodology is fast and efficient and is quite suitable for high speed production. Moreover, the RIM process has the capability to reduce or eliminate shrinkage related defects in contact lenses. It permits the use of IPN's for the preparation of hydrogels suitable for contact lenses which have properties that are not achievable either from acrylic polymers or urethane polymers separately.